This invention relates generally to solar cells, and more particularly the invention relates to metal contact structures for use in solar cells.
The use of photovoltaic cells for the direct conversion of solar radiation into electrical energy is well known, see Swanson, U.S. Pat. No. 4,234,352 for example. Briefly, the photovoltaic cell comprises a substrate of semiconductive material having a p-n junction defined therein. In the planar silicon cell the p-n junction is formed near a surface of the substrate which receives impinging radiation. Radiated photons create mobile carriers (holes and electrons) and the substrate which can be directed to an electrical circuit outside of the cell. Only photons having at least a minimum energy level (e.g., 1.1 electron volt for silicon) can generate an electron-hole pair in the semiconductor pair. Photons having less energy are either not absorbed or are absorbed as heat, and the excess energy of photons having more than 1.1 electron volt energy (e.g., photons have a wavelength of 1.1 μm and less) create heat. These and other losses limit the efficiency of photovoltaic cells in directly converting solar energy to electricity to less than 30%.
Solar cells with interdigitated contacts of opposite polarity on the back surface of the cell are known and have numerous advantages over conventional solar cells with front side metal grids and blanket or grid metallized backside contacts, including improved photo-generation due to elimination of front grid shading, much reduced grid series resistance, and improved “blue” photo-response since heavy front surface doping is not required to minimize front contact resistance and since there are no front contacts. In addition to the performance advantages, the back/contact cell structure allows simplified module assembly due to coplanar contacts. See Swanson U.S. Pat. No. 4,927,770 for example.
The present invention is directed to an improved metal contact structure which is especially applicable to solar cells.